1. Field of the Invention
This invention relates to the field of electromagnetically actuated clutches and more particularly pertains to such clutches that use friction clutch elements to make the connection between the driving and driven members.
2. Description of the Prior Art
In the prior art various methods have been used to produce engagement between the driving and driven members of a clutch when an electrical coil is energized. For example, the driving member may be formed with a cylindrical hub adapted to be coupled to the driven member when a shoe is contracted radially into frictional gripping engagement with the hub. The shoe is often surrounded by a helically coiled spring having one end connected to the driven member and having an opposite end or armature end that is moved by magnetic force axially into engagement with the rotatable driving member when an electromagnet is excited. In a clutch operating on this principle, pilot torque is transmitted magnetically between the driving member and the armature end of the spring to turn the armature end of the spring through a short distance. This movement causes the coils of the spring to wrap around the shoe and to contract the shoe into frictional gripping engagement with the hub of the driving member. The shoe, therefore, rotates with the hub and transmits torque to the driven member. The pilot torque transmitted magnetically between the driving member and the armature end of the spring is relatively small and is transferred directly through the spring to the driven member. The pilot torque is amplified by the action of the spring wrapping downwardly around and contracting the shoe; therefore a high percentage of the total transmitted torque is transfered between the driving and driven members by way of the shoe.
Clutches of this general type are disclosed in U.S. Pat. Nos. 3,149,706; 3,177,996; 3,685,622; 4,201,281; 4,273,226; 4,377,224 and 4,445,606.
The clutch described in U.S. Pat. No. 817,210, is selectively actuated by energizing an electrical coil that moves brake shoes into engagement with a friction surface.
In order to improve the torque capacity in clutches of the type that employ electrical energy to produce frictional engagement without amplification, it is necessary that the material of the clutch have high permeability, that the electrical current be increased, that the number of windings in the coil be increased or that the number of magnetic poles be increased. Unfortunately, these changes substantially increase the weight and cost of the clutch and require larger electrical current.
The weight of an electromagnetically actuated friction clutch can be minimized more efficiently by mechanically augmenting the force produced by the electromagnetic that applies the brake shoes to the friction surface on which the driving engagement occurs.